Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 646 PM EDT Fri Apr 03 2020 Valid 12Z Tue Apr 07 2020 - 12Z Sat Apr 11 2020 ...Guidance/Predictability Assessment and Weather/Threats Highlights... One feature with ongoing guidance uncertainty is a potentially quite concentrated storm system that may track into the Gulf of Alaska by around day 5 Wed. This system may be accompanied by strong winds along with a period of heavy precipitation focused most likely over the Panhandle but possibly extending back to the southeastern coast of the mainland depending on low track. Dynamic support aloft comes from shortwave energy forecast to be over the eastern Aleutians/Alaska Peninsula on Tue. GFS/ECMWF runs through the 12Z cycle have been fairly consistent in principle. Weaker ensemble means had been similar before the 12Z cycle. The 12Z GEFS mean came in with a slower shortwave and in fact phased with the mid-latitude upper low to the south in a way not seen in other guidance, so that solution appears to be a low-probability one at this time. CMC/UKMET and other models vary widely. Preference is with a 12Z ECMWF/GFS blend tilted somewhat more toward the ECMWF. This provides reasonable continuity for overall evolution along with a deeper trend from yesterday. Behind this system, guidance evaluation focuses on the strength/progression of upper ridging that builds into the North Pacific/Bering Sea and then possibly the mainland--and in turn eastward/northward extent of low pressure reaching the western Aleutians by Wed-Thu and the system initially over the mid-latitude Pacific. Latest GFS runs are among the most progressive for the overall pattern from the Aleutians through the Bering/Sea and mainland. While the 12Z GEFS mean is suspect farther east, it does provide support for the idea that the western Aleutians system should stall rather than continue onward per the GFS. Meanwhile operational ECMWF runs stray a bit to opposite sides of the spectrum by the latter half of the period, the old 00Z run being strong/slow/closed with upper ridging and suppressed with the overall surface pattern versus the new 12Z run that lets the upper ridge give way enough for the mid-latitude Pacific system to reach northward into the Aleutians by late next week. In fact 06Z/12Z GFS also varied greatly for the mid-latitude low but in opposite sequence of ECMWF runs. The guidance spread/variability led to the ECMWF/GFS blend used for the first couple days of the period being transitioned toward a solution comprising half 00Z ECMWF mean and the rest a split between the last two ECMWF runs for days 7-8 Fri-Sat. The new 12Z ECMWF mean seems to provide support for an intermediate solution, perhaps a bit more upper ridge progression than derived from the forecast blend but still a fair degree of suppression at the surface. Multiple surface highs tracking over the mainland will bring below normal temperatures for a majority of the period. The North Slope region should be the one exception, tending to see above normal readings most days. Uncertain pattern progression late in the period will determine how much moderation may eventually occur over western areas. The western Aleutians should see some precipitation with the system tracking into the area. How much moisture reaches the rest of the Aleutians and Peninsula will be sensitive to the uncertain aspects of flow aloft by the latter half of the week. Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Tue-Thu, Apr 7-Apr 9. - High winds across portions of the Alaska Panhandle and mainland Alaska, Tue-Wed, Apr 7-Apr 8. WPC medium range Alaskan products including 500mb, surface fronts/pressures progs and sensible weather grids can also be found at: https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html